1. Field of the Invention
The present invention relates generally to lithographic projection apparatus and more particularly to lithographic projection apparatus including an acoustic sensor.
2. Description of the Related Art
This application claims priority from European Patent Application EP 00310409.8, herein incorporated by reference.
A typical lithographic projection apparatus includes a radiation system for supplying a projection beam of radiation, a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern, a substrate table for holding a substrate, and a projection system for projecting the patterned projection beam onto a target portion of the substrate.
The term “patterning structure” as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-adressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of integrated circuit devices will be present on the substrate (wafer). These integrated circuit devices are then separated from one another by a technique such as dicing or sawing, whence the individual integrated circuit devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
Unless otherwise specified, the term “projection beam” in the present specification and claims encompasses both a patterned projection beam downstream of the patterning structure and a non-patterned projection beam (either in the absence of a pattern or in the absence of patterning structure) either upstream or downstream of the location of the patterning structure.
In a lithographic projection process it is important to control accurately the dose (i.e. amount of energy per unit area integrated over the duration of the exposure) delivered to the resist. Known resists are designed to have a relatively sharp threshold whereby the resist is exposed if it receives a dose above the threshold but remains unexposed if the dose is less than the threshold. This is used to produce sharp edges in the features in the developed resist even when diffraction effects cause a gradual tail-off in intensity of the projected images at feature edges. If the projection beam intensity is too incorrect, the exposure intensity profile will cross the resist threshold at the wrong point. Dose control is thus crucial to correct imaging.
In a known lithographic apparatus, dose control is done by monitoring the projection beam intensity at a point in the radiation system and calibrating the absorption of radiation of the projection beam that occurs between that point and substrate level. Monitoring the projection beam intensity is performed using a partially transmissive mirror in the radiation system to divert a known fraction of the projection beam energy to an energy sensor. The energy sensor measures the radiation energy in the known fraction of the projection beam and so enables the projection beam energy at a given point in the radiation system to be determined. The calibration of said absorption of radiation is done by replacing the substrate by a supplementary energy sensor for a series of calibration runs. The output of the former energy sensor effectively measures variations in the output of the radiation source and is combined with the calibration results of said absorption to predict the energy level at substrate level. In some cases the prediction of the energy level at substrate level may take account of, for example, settings of components for shaping a cross section of the projection beam of radiation. Parameters affecting the dose, e.g. duration of the exposure or scanning speed, and/or the output of the radiation source can then be adjusted to deliver the desired dose to the resist.
While the known method of dose control takes account of variations in the output of the radiation source and deals well with predictable variations in absorption of radiation occurring downstream of said partially transmissive mirror, not all variations in absorption are easily or accurately predictable. This is particularly the case for apparatus using exposure radiation of wavelengths such as 157 nm, 126 nm or EUV (less than 50 nm, e.g. 13.6 nm), where the use of a shorter wavelength is essential to reduce the size of the smallest features that can be imaged. Such wavelengths are heavily absorbed by air and many other gases, so that lithographic apparatus making use of them must be either flushed with non-absorbing gases or evacuated. Any variations in the composition of the flushing gas, or leaks from the outside, can result in significant and unpredictable variations in the absorption of the projection beam radiation occurring downstream of the energy sensor in the radiation system and hence of the dose delivered to the resist.